The constantly increasing demands for pollution reduction have recently led to the introduction of concepts involving a lean fuel mixture and concepts involving exhaust recirculation, among others. Lean fuel mixture concepts have the advantage of making fuel economy is possible, in addition to leading to a reduction of raw emissions. With exhaust recirculation concepts, when a 3-way catalytic converter is used at the same time, a particularly good reduction of overall emissions is possible.
It is common to both concepts that in certain operating ranges of the engine, it is desirable to use a fuel which is leaned down as much as possible and to establish a high exhaust recirculation rate, respectively, while necessarily maintaining a certain margin from the so-called "running limit." The "running limit" can be defined as the limit of the leaning down and/or or of the exhaust recirculation rate beyond which ignition of the mixture no longer occurs reliably for every piston working cycle, where an acceptable running smoothness of the engine does not occur, or where exhaust emissions begin increasing again because of insufficient combustion. Because of the need to maintain a required margin from this running limit, the potential of these two concepts, that is, the potential of using lean fuel mixtures and of recirculating the exhaust, is not completely utilized.
Principles for recognizing the running smoothness of the engine are disclosed in DE-A-29 06 782 for lean regulation by using a rotational irregularity sensor, in DE-A-33 15 048 by means of structure-borne sound sensors, and in DE-A-33 14 225 via an exhaust volume flow measurement, as well as in other prior publications. Recognizing the "running limit" by making use of rotational irregularity can be used principally in engines that are on the test bench. However, the above cannot be accomplished in the case of engines disposed in production vehicles, since uneven spots in the road, which lead to misleading signals, are fed in via the drive train.
In practice, the recognition of the "running limit" in a production vehicle via structure-borne sound sensors is hardly usable either. Besides possible input resulting from uneven spots in the road, there are also a wealth of interfering signals, which lead to problems of misinterpretation because of an extremely poor signal-noise ratio, which is due to the relatively low useful signal that can be detected.
Additionally, an analysis of the exhaust volume flow is relatively complicated to perform and, consequently, cannot be introduced in production vehicles at least for cost reasons.
The use of combustion chamber pressure sensors for recognizing the running limit has previously been intensively employed in the development of engine tuning. The tuning is performed each time such that a distinct margin from the lean running limit is maintained. With the availability of fairly inexpensive but quite precise combustion chamber pressure sensors, this method has meanwhile been introduced even in production vehicles, by providing lean regulation. References to the above are found in publications SAE 930882 and 930351 of the 1993 SAE Congress in Detroit. In the process proposed in the above publications, an estimate of the effective torque is derived from the combustion chamber pressure. In addition, the pressure values of certain positions of the crankshaft are estimated and from the same, an estimate of the internal work performed by the engine is derived. The above process relies on the process for tuning engines which is practiced on the test bench, and in which the so-called indicated mean effective pressure, i.e. the contour integral of the pressure, is determined via the cylinder volume. A measure for the running smoothness is derived via statistical methods from the above internal work or from the estimated moment introduced. In the above process, however, knowledge of the crankshaft state is critical to the derivation of the current volume of the combustion chamber from the respective structural data of the relevant engine. For the above reason, in mass production of running limit recognition according to this method, the signal of the combustion chamber pressure transducer is supplied to a processing circuit, which receives information about the state of the crankshaft from a crankshaft angle marking sensor as a further input signal. The processing circuit is integrated into the engine control electronics remote from the sensor, since the crankshaft angle marking transducer signals are also available at that location. As a result, however, each manufacturer encounters the necessity of specifically implementing an algorithm in the engine electronics that perform the corresponding evaluation. If this recognition is realized with software in an already existing processor of the engine control electronics, then this software function can be integrated into existing engine control software only at a high cost. In order to avoid the above, a second processor must be employed, which, however, require a redesign of all the electronic hardware of the engine.